This invention relates to adhesives and to methods for making such adhesives. In one embodiment, the adhesive is a pressure sensitive adhesive. These adhesives are comprised of microgels of acrylic polymer covalently bonded to each other and optionally to a non-gelled polymer through an interlinking polymer. In one embodiment, these adhesives are characterized by high peel energy, high tack and high shear holding power.
Pressure sensitive adhesives (PSAs) are viscoelastic-elastomeric materials that can adhere strongly to a variety of substrates upon application of light contact pressure and short contact time. PSA applications range from various tape and label products used for industrial, medical and consumer markets. In these applications, PSAs are normally coated onto face or backing materials at 15-100 xcexcm dry film thickness. Typical face stock materials are paper, film and foil. The PSA coated face stocks may be printed and used as labels or tapes.
Polymers used in making PSAs are primarily divided into styrenic block copolymer (SBC) and acrylic. SBCs are mainly used for paper label and Oriented Polypropylene (OPP) tapes due to their highly aggressive tack and moderate shear holding power. SBCs exist in a two phase morphology, the first a hard polystyrene domain residing in the second, a polyisoprene rubber matrix. This morphology allows the SBC to be compounded with a high loading of hydrocarbon tackifiers to increase tack without significantly reducing shear. This is possible because the tackifier is compatible only with the polyisoprene phase.
Acrylics are mainly used in film and specialty paper labels because of their optical clarity, oxidative and UV-stability, and resistance to bleeding or migration into the face stock material. Unlike SBCs, acrylics are inherently tacky due to their high entanglement molecular weight (Me) without the need of adding tackifier. The monomers most frequently used in making acrylic PSAs are 2-ethyl hexyl acrylate (2EHA), n-butyl acrylate (n-BA) and iso-octyl acrylate (i-OA). A variety of comonomers such as acrylic acid (AA), methyl acrylate (MA), vinyl acetate (VA), styrene (Sty), methyl methacrylate (MMA), dioctyl maleate (DOM), and others have been used to tailor the adhesive performance.
Within the acrylic family, PSAs can be divided into solvent borne, emulsion, suspension and hot melt. Solvent borne acrylic PSAs may be prepared by semi-continuous solution polymerization of the above monomers using an oil soluble initiator. Aluminum acetylacetonate (AAA) crosslinker in solution may be post added and the film cast undergoes a crosslinking reaction after the solvent evaporates. Acrylic emulsion PSAs have been prepared by semi-continuous emulsion polymerization of the above monomers using water soluble thermal or redox initiators to form a polymer late. It is common for acrylic emulsion PSA to contain a high level of microgels as a result of chain transfer to polymer reactions during polymerization. Batch suspension polymerization may be used to prepare 100-200 xcexcm acrylic polymer particles suspended in water. These microspheres when coated at low coat weight (ca. 15 g/m 2) form discontinuous film which provide excellent PSA removability on various substrates (e.g., Post-It Notes(copyright)). Acrylic hot melts may be prepared by solution polymerization of the above monomers along with small amount of UV-reactive comonomer. The solvent is subsequently stripped and the 100% solid molten polymer cast onto a face stock and UV-cured to build cohesive strength.
Environmental regulations such as the Clean Air Act have placed strict restrictions on VOC emissions. Thus, the use of solvent-borne acrylic PSAs has declined steadily for the past ten years. Despite the continuing decline, acrylic solution PSAs are still in use today for high performance applications requiring high tack, peel and excellent shear holding power. Furthermore, acrylic solution PSA is still the choice for clear film label application requiring pasteurization.
In general, there are two major weaknesses of acrylic emulsion PSAs which limit their usage compared to their solvent counterparts. First, for the same degree of crosslinking or gel content, acrylic emulsion PSA film has much lower shear holding power compared to that of solvent-based acrylic PSAs. Second, acrylic emulsion film, due to the surfactant presence, becomes opaque after exposure to water vapor. This problem also is known as xe2x80x9cwater whiteningxe2x80x9d, and occurs in emulsion cast PSA films.
There are several reasons that have been suggested for the inferiority of acrylic emulsion PSA films compared to their solvent counterparts in terms of their loop tack, peel and shear holding power. Incomplete latex film formation of poly(2-ethyl hexyl methacrylate) [P(2EHMA)] caused lower viscoelastic energy dissipation which brought lower peel compared to that of solution cast film. Surfactant migration onto the film-substrate boundary layer was shown to reduce peel. However, various studies have shown that the thick surfactant boundary layer could be avoided and perhaps the peel could even be increased if the surfactant used was a plasticizing type. An example of plasticizing type surfactant is the nonyl phenol ethoxylates which are used widely in PSA lattices. Starting in the early 1980""s, large tackifier suppliers such as Hercules, Arizona Chemical and AKZO Nobel began dispersing some tackifiers in water to form 0.2-1 xcexcm particle diameter dispersions. These tackifier dispersions could be blended at 15-30 wt. % with acrylic emulsions to increase the PSA film tack and peel. Tackified acrylic emulsion PSAs have enjoyed significant market growth for the past ten years in paper label applications. However, although both solvent and water borne acrylic PSAs are derived from the same monomers, the adhesive (peel and tack) and cohesive (shear holding power) properties of emulsion PSAs are inferior to those of solvent PSAs.
In order to increase the peel energy of emulsion acrylic PSAs, aqueous tackifier dispersion was added at 15-25 wt. % into the base latex. See, for example, U.S. Pat. No. 5,623,011. There are numerous problems created by this technology due to a lack of understanding of the emulsion film formation and the formation of the network morphology. Build-up on the die during high speed rotary die cutting process was found as well as significantly lower shear holding power when compared to that of the neat emulsion. More surprisingly, peel energy from low surface energy substrates such as high density polyethylene (HDPE) or low density polyethylene (LDPE) showed little improvement compared to that of the neat emulsion.
U.S. Pat. No. 4,144,157 discloses a self-adhesive composition comprising a crosslinked copolymer based on (a) 0.01-5 wt % of acrylic or methacrylic [(2-alkoxy-2-phenyl-2-benzoyl) ethyl] esters as photoinitiators with (b) at least one monomer capable of producing a self-adhesive polymer, in which the composition is crosslinked by exposure to UV radiation. The polymerization of the photoinitiator (a) with the monomer (b) may be by emulsion polymerization.
U.S. Pat. No. 4,474,923 discloses self-curable latex compositions which contain particles of an oxazoline-modified polymer and particles of a coreactive polymer. These latex compositions are curable, yielding films and other articles which are described as having good tensile and elongation properties and excellent resistance to water and aqueous fluids. These latex compositions are self-curing at room temperature.
U.S. Pat. No. 5,164,444 discloses emulsion PSA formulations comprising alkyl acrylates copolymerized with unsaturated carboxylic acids, vinyl esters and diesters of dicarboxylic acids and, in addition to these basic ingredients, may also include a chelating monomer, such as acetoacetoxy ethyl methacrylate, which enables complex formation with metal salts. U.S. Pat. No. 5,189,126 discloses an emulsion PSA including the same basic ingredients as the ""444 patent plus small amounts, i.e., 0.1 to 1 percent, of a reactive multifunctional monomer having a cyanurate or phosphate functionality. The emulsion PSAs of both the ""444 and the ""126 patents may also include a reactive surfactant, such as sodium vinyl sulfonate or sodium styrene sulfonate, which polymerizes and becomes part of the emulsion polymer, and may be ionically cross-linked by metal salts such as aluminum acetate. Both the ""444 and the ""126 patents disclose that while the multifunctional monomer provides some improvement in the shear performance, it still may be necessary to cross-link the polymer by complex formation with the metal salt. The ""444 patent teaches that the chelating monomer may alter the gel content and/or glass transition temperature of the resulting PSA. Similarly, the ""126 patent teaches that adding multifunctional monomers to the copolymer may alter the gel content and/or glass transition temperature of the resulting PSA.
Although the foregoing emulsion PSAs represent a significant advance over previously known PSAs, the formulations have continued to be developed in an effort to provide the desired adhesive features of peel and tack, and the cohesive property of shear holding power. U.S. Pat. No. 5,623,011 discloses an emulsion polymer formed from a monomer mix comprising 35 to 60% by weight alkyl acrylates, 15 to 35% by weight vinyl esters, 15 to 35% by weight diesters of dicarboxylic acids, and up to about 5% by weight unsaturated carboxylic acids, and (b) a tackifier having an acid number of from about 30 to about 60 and a ring and ball softening point of from about 50xc2x0 C. to about 70xc2x0 C. In one embodiment, the tackifier has a hydrocarbon resin component and a rosin-based resin component, with the preferred tackifier comprising an aromatic modified aliphatic resin containing wood rosin. A preferred adhesive comprises 8 to 12 parts by weight tackifier per 100 parts by weight polymer.
U.S. Pat. No. 5,723,191 discloses tackified PSAs comprising a copolymer based on an acrylic backbone, with a glycidyl monomer, unsaturated carboxylic acid monomer, preferably a vinyl lactam monomer, and a tackifier, in which the adhesives are dual curable and are prepared in a mixture of organic solvents.
This invention relates to an adhesive composition comprising: microgels of at least one acrylic polymer; and an interlinking polymer covalently bonding at least some of said microgels to each other through said interlinking polymer. In one embodiment, the adhesive further comprises at least one non-gelled polymer, said interlinking polymer covalently bonding at least some of said microgels to said non-gelled polymer.
In one embodiment, the invention relates to an aqueous dispersion, comprising: water; microgels of at least one acrylic polymer dispersed in said water; and at least one interlinking polymer capable of covalently bonding said microgels to each other through said interlinking polymer. In one embodiment, the dispersion further comprises at least one non-gelled polymer, and the interlinking polymer is capable of covalently bonding said microgels to said non-gelled polymer. This invention also relates to an adhesive composition made by heating the foregoing aqueous dispersion at a sufficient temperature and for an effective period of time to covalently bond at least some of said microgels to each other through said interlinking polymer, and optionally covalently bond at least some of said microgels to the foregoing non-gelled polymer.
In one embodiment, the adhesive is a pressure sensitive adhesive. These adhesives may be characterized by high peel energy, high tack and high shear holding power.
In one embodiment, this invention relates to a process for making an adhesive, comprising:
(i) forming a monomer emulsion comprising water, at least one acrylic monomer, optionally at least one comonomer, an unsaturated carboxylic acid, optionally a chain transfer agent, and an interlinking agent;
(ii) mixing a polymerization initiator with said monomer emulsion and polymerizing the resulting mixture to form an aqueous dispersion comprising microgels of at least one polymer; and
(iii) heating the dispersion formed in step (ii) at a sufficient temperature and for an effective period of time to covalently bond at least some of said microgels to each other through said interlinking agent.
In one embodiment, this invention relates to a process for making an adhesive, comprising:
(A) forming a first aqueous dispersion comprising microgels of at least one polymer dispersed in water by
(i) forming a first monomer emulsion comprising water, at least one acrylic monomer, optionally at least one comonomer, optionally a chain transfer agent, and an unsaturated carboxylic acid; and
(ii) mixing a polymerization initiator with said first monomer emulsion and polymerizing the resulting mixture to form said first aqueous dispersion;
(B) forming a second aqueous dispersion comprising a non-gelled polymer by
(i) forming a second monomer emulsion comprising water, at least one acrylic monomer, optionally at least one comonomer, an unsaturated carboxylic acid, optionally a chain transfer agent, and an interlinking agent;
(ii) mixing a polymerization initiator with said second monomer emulsion and polymerizing the resulting mixture to form said second aqueous dispersion;
(C) combining the first and second aqueous dispersions to form a combined aqueous dispersion; and
(D) heating the combined aqueous dispersion at a sufficient temperature and for an effective period of time to covalently bond at least some of the microgels to at least some of the non-gelled polymer.